CHAPTER 1 INTRODUCTION

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1 1 CHAPTER 1 INTRODUCTION 1.1 GENERAL In telecommunication network, technology and system requirements are changing rapidly over recent years. More and more, the researchers in telecommunication network attempt to provide advanced technological services in an attractive and usable way to meet the demand for high-capacity long-haul telecommunication systems with very low data loss. This demand can only be achieved with a wide system bandwidth which can support high bit rate data. Increasing communication bandwidth with reduced loss will be the focus of attraction in future telecommunication systems (Sharam Hekmat 2005). Advanced changes in telecommunication has invariably invaded the devices that assist the communication process such as mobile phones, land-phones, intercoms, automatic voice answering systems, pagers, radar systems, radio and television. There is also a vast array of networks, which connect these devices including computer networks, public telephone networks, radio networks and television networks. Of the many different types of communication channels available, optical fibers have proved to be the most promising technology. In late 1970s, optic fiber began to replace coaxial cable as the transmission medium for information sharing in telecommunication networks. Technology evolution of optical networks which is capable of supporting national and global communication requirements for the 21st century evolved

2 2 from the invention of laser by Schawlow and Townes in 1958, followed by the work of Kao and Hock ham on optical fibers in In 1983, creation of Internet with Transmission Control Protocol/Internet Protocol (TCP/IP) and World Wide Web (WWW) in 1993 ignited the growth of data traffic on the network, and by the year 2002, the amount of data traffic in the network exceeded that of voice traffic. Data traffic grew from 3 billion to 24 billion gigabytes since the year New requirements from scientific users are driving towards the deployment of high-performance optical networking in addition to the increased expectation from residential and business users (Michael O Mahony et al 2006). 1.2 OPTICAL COMMUNICATION NETWORKS In early 1980s, a revolution in telecommunication networks initiated the invention of a promising technology referred as Optic Fiber Cable (OFC) technology. Optical fiber is used by many telecommunication companies to transmit data signals in long-distance and high-demand applications. Optical fiber has large advantages over existing copper wire due to its low attenuation and high resistance from interference. Also, developing an infrastructure in a city could only be achieved with greater difficulty and it also consumes more time. Fiber-optic networks could be a complex system and involves huge cost to install and operate. Viewing these constraints, the optical fiber communication systems are suggested mainly for long-range communications wherein we could extract optimum capacity from the system for the cost invested on it. However, in recent years, the dropping of prices for fibers and its accessories to establish the communication system has contributed significant impact in the rapid utilization of fibers than copper for communication even in home, commercial and professional applications. Prices for fiber have dropped to $850 (46559 INR) per subscriber in United

3 3 States of America and comparatively lower in other countries, where trenchdigging costs are low. Since 1990, when optical-amplification systems became commercially accessible in the telecommunication industry, they set a wide network of inter-city and trans-oceanic fiber communication lines. An intercontinental network with a long-range of 250,000 km submarine communication cable with a capacity of 2.56 Tbps was laid in the year According to telecommunication investments report, there has been a dramatic increase in network capacity since 2004 (Ron C. Focazio 2012). As a solution to meet-out the demand for higher capacity networks with reduced cost and wider bandwidth requirements, network providers stepped forward to optical networks (Zouganeli 2006 and Suzuki et al 2003). Following are the factors, which drives the need for optical networks: Fiber Capacity Restoration Capability Reduced Cost Wavelength Services Fiber Capacity Data transmissions were done initially with copper as a medium of conduction. However, due to extensive requirement for channel capacity, the researchers concentrated on how to utilize the existing channel optimally. Even, the Time Division Multiplexing (TDM) could not serve the requirements. Later, Frequency Division Multiplexing (FDM), Wavelength

4 4 Division Multiplexing (WDM) and Dense Wavelength Division Multiplexing (DWDM) were invented. Even then, the technology could not serve the everincreasing demand for bandwidth. The fiber is now considered to be one of the best choices for communication due to its inherent favorable properties. The wide bandwidth available with the fiber will fulfill the ever-increasing demand if it is properly channelized Restoration Capability In general, to restore the network from failures, network planners use more network elements to increase fiber capacity. In optical networks, it is found to be one of the important parameter in determining the performance of the network. Further, a simple cut in the fiber could cause multiple failures in the entire communication system. The property of the fiber to restore from failure is also an added advantage for its vast utilization Reduced Cost In optical WDM network, the wavelengths that add to or drop from the network traffic at a site need the corresponding electrical nodes rather than placing a network element for each channel to de-multiplex the signals. Other channels can simply pass optically through WDM network, which avoids high cost of electronic cross-connects, and hence the network management is simplified to a larger extent Wavelength Services Another important aspect for the use of fiber is its capability to resell bandwidth and thereby one can generate revenue from the optical networks. Service providers can lift up the revenue by vending the

5 5 wavelengths. This service allows the customer to utilize the bandwidth equal to that of a dedicated fiber. 1.3 WAVELENGTH DIVISION MULTIPLEXING In high speed long-haul network backbones, WDM has been proven to be a successful technique which allows hundreds of independent light-paths multiplexed through a single fiber, equivalent to an effective bandwidth of 1 Tbps (Pin-Han Ho 2004). Also, it offers the potential to deliver huge capacities by multiplexing hundreds of independent channels through a single fiber. Through a single fiber, optical WDM networks are capable of transporting hundreds of wavelengths at the rate of 10 Gbps per wavelength (Muchanga et al 2006) and even more nowadays. Due to unique features of WDM networks, future optical networks will also require the WDM transport layer to deliver automatic wavelength provisioning and optical restoration (Tang 2006). As WDM networks carry huge volume of traffic, survivability issue in high bandwidth WDM networks has become an important research issue in recent years. WDM divides the tremendous bandwidth of fiber into many non-overlapping wavelengths (WDM channels), which can be operated at any desirable speed. The wavelengths can be individually routed through a network or individually recovered by wavelength-selective components. WDM allows us to use much of the fiber bandwidth, although various devices, systems and network issues will limit the utilization of the full fiber bandwidth (Mukherjee 2000). 1.4 FAULT MANAGEMENT IN OPTICAL WDM NETWORKS Today, network failure is the most probable cause of congestion, packet loss and has notable impact on maintaining high Quality of Services (QoS). Communication networks face intentional interruptions due to wide

6 6 variety of failures such as natural disasters, wear out and overload, software bugs and human errors (Pickavet et al 2006). Due to more number of channels involved in a single fault, detecting and isolating faults becomes even more complex (Machuca et al 2003). Therefore, network survivability is an important and difficult issue involved in the deployment of QoS scheme in optical networks (Soung Liew et al 1994 and Yu Liu et al 2005). The optical network is developed and being instigated on existing Synchronous Optical Networking (SONET) architecture, which provides its own restoration and protection schemes. To guarantee the stability of restoration scheme between the electrical and optical layer, an intelligent network management system is needed. In addition, the network management system will able to identify or prevent the possible conflicts. Network management system must also be able to monitor signal performance for every wavelength. The adding up of optical add/drop multiplexers and Optical cross-connects (OXCs), lead to degradation in the end-to-end performance of wavelengths. As the number of wavelength on each fiber increases, it is necessary to have an intelligent method to monitor all. The ability to manage and provide new services to customers quickly is crucial. Provisioning of end-to-end services can be difficult, especially when network capacity increases. Service providers need an intelligent network management system to manage the end-to-end wavelength services thereby increasing their bandwidth revenues. As WDM networks carry huge volume of traffic, survivability issue has become a key issue in research and development. In optical networks, it is very important to have a mechanism that guarantees quick detection, isolation and localization of failures and efficient recovery.

7 7 Provisioning of light-path can be either static or dynamic. In static light-path provisioning, light-paths are pre-established, whereas, light-paths are dynamically established in dynamic provisioning which adapt to the dynamic change in client traffic load (Chunsheng Xin 2007).Static (or) proactive protection and Dynamic (or) reactive restoration are the main approaches employed to ensure survivability in optical WDM networks. Approaches to ensure network survivability are given in Figure 1.1. Figure 1.1 Several approaches for network survivability Path protection/restoration In path protection scheme, backup routes and resources are reserved during connection setup itself, while in path restoration, backup routes and resources are discovered dynamically only after the link failures. When a link fails, the information about the failure is informed to the source and the destination node of each connection along the failed link through the messages from the nodes adjacent to the failed link.

8 Link protection/restoration In link protection scheme, backup routes and resources are reserved around each link during connection setup itself, while in link restoration, the end nodes dynamically discover a route around the link after any failure in the link. In optical networks, protection and restoration techniques highlight the following aspects for enhancing the scalability of the network (Banerjee et al 2001): Fault isolation Fault localization Fault notification and Fault mitigation The protection scheme handles fault recovery by computing a backup light-path for every connection request, in addition to the primary light-path. The resources along the backup light-path are reserved and will be used in the event of failure in the primary light-path. If both primary and backup light-paths can be found for the corresponding demand, the demand is accepted. If one could guarantee on 100% restoration for any single failure, the high resource redundancy usually results in proactive protection and poor network throughput. On the other hand, a backup light-path is searched only after the interruption, if any, in the primary light-path with the restoration scheme. Although restoration scheme is efficient in terms of capacity utilization and blocking probability, it may lead to an unacceptable long restoration time due to its dynamic discovery for a backup light-path after failures.

9 9 1.5 VARIOUS FLOODING ISSUES IN OPTICAL NETWORKS Update of network state information is the basic function of network management which includes aggregated load and topology information. Flooding is the simple broadcast algorithm in which every node broadcasts the message when it receives for the first time. In flooding approach, each node receives the message from all its neighbours in a collision-free network. For a network with N routers, any change in the network topology can generate Link State Advertisement (LSA) messages of the order of O(N 2 ), known as the LSA N-squared problem. When N is large, the LSAs generated can degrade the network performance significantly and introduce routing instabilities (Alfred and David 2000). In all-optical network, information regarding the resource and network topology needs to be kept up-to-date, so that the correct light-path decision can be made. All flooding method is a flooding technique where each channel status change is flooded. A change in channel status happens when an available channel becomes occupied or vice versa. This simple scheme guarantees that a flooding message can reach all nodes if there is no collision and the network is connected. When the message is received by the recipient for the first time, the recipient re-broadcasts it. This flooding method suffers from the problems of excessive redundancy of messages, resource contention and signal collision. Also, it causes high protocol overhead and frequent interference with the existing traffic in the networks. Instead of flooding with every change in the channel status, the network can hold the process until it reaches a certain point. This method is referred to as Lazy Flooding (Caixia chi et al 2007).

10 RESOURCE OPTIMIZATION IN OPTICAL WDM NETWORKS To optimize network utilization, it is very important to maximize the carried traffic while guaranteed with the QoS. On one hand, high class services might require full reliability and bounded recovery time to provide deterministic service availability. On the other hand, low class services do not impose such constraints. Further, they can tolerate low reliability and slow recovery time. In a connection-oriented scenario, network spare resource allocation depends mainly on different connection classes carried by the network and failure scenario. For example, high-class connections might require that backup routes and resources are reserved along the path to obtain full reliability and recovery time while low class connection may instead utilize dynamic restoration (Nicola Andriolli et al 2005 and Nicola Andriolli et al 2007). Resources can be optimized both in failure and failure-free conditions. Since the scope of this research is limited to failures in WDM networks, the ultimate goal is to achieve uniform and balanced utilization of resources in failure conditions. 1.7 ROUTING AND WAVELENGTH ASSIGNMENT IN OPTICAL NETWORKS In WDM networks, data are typically routed from source node to destination node through one or more wavelengths associated with a lightpath. After the connection is terminated, the wavelengths become available and can be used for future connections. Since light-paths are the basic building block of network architecture, connection establishment for a session request is very important which involves selection of route and wavelength assignment along this route (Jonathan Lang et al 2001, Asuman Ozdaglar and

11 11 Dimitri Bertsekas 2003). This problem is called Routing and Wavelength Assignment (RWA) problem. Wavelength selection plays a vital role in the network performance. To solve the sub-problems of wavelength assignment, various heuristic algorithms such as First Fit (FF) wavelength assignment, Random Fit (RF) wavelength assignment, Most Used (MU) and Least Used (LU) wavelength assignment schemes are used (Ramesh Kancheti et al 2006) which are explained in the following sub-sections First Fit (FF) wavelength algorithm In this wavelength assignment strategy, the list of used and free wavelengths is maintained and a numerical index is predefined with each free wavelength. FF strategy always chooses the lowest indexed wavelength from the list of free wavelengths and assigns it to the connection request. When the request is completed, the wavelength is added back to the free wavelength set with the original index value. The disadvantage of this approach is that the lower indexed wavelengths are much more utilized than the higher indexed wavelengths. Hence, the higher indexed wavelengths are underutilized. Since all the nodes in the network use the lower numbered wavelengths, contention for the higher indexed wavelengths increases which results in higher connection drop rate in the network Random Fit (RF) wavelength algorithm RF algorithm determines available wavelengths and then chooses the wavelength randomly amongst the available set of wavelengths. Even though the RF wavelength assignment performs better than the FF assignment algorithm, it can choose any of the free wavelengths and it lacks a definite approach for wavelength assignment. As a result, the random fit wavelength algorithm does not yield good results.

12 Most Used (MU) and Least Used (LU) wavelength algorithm Under the MU wavelength strategy, when a wavelength request is received, the one that is used on the greatest number of fibers in the network is allotted. If more than one wavelength has the same maximum usage, the first fit wavelength algorithm is used to break the tie. The LU wavelength assignment also works similar to the most used strategy except the least used wavelength is assigned. Both MU and LU wavelength assignment algorithms need the global knowledge of the network Round Robin (RR) wavelength algorithm In RR wavelength assignment, the wavelengths are indexed. When a request for a wavelength is received, the first indexed wavelength is assigned and for every subsequent request, the node assigns the next higher numbered wavelength. This process continues until the highest indexed wavelength is assigned. When all the wavelengths have been assigned, the first indexed wavelength is assigned to the next request. The process thus continues in a round robin fashion. In this manner, all the wavelengths are utilized equally which reduces the blocking probability considerably. Blocking probability in the network increases when the requested wavelength is not available for any connection request. Among the wavelength assignment strategies, FF and RF techniques are simple to implement. They are used most practically since they depend on the state of the node at that instant and choose the wavelength from the set of free wavelengths. However, they will not yield optimum results as they are unaware of the state of the network. In RR wavelength assignment approach, the light-path can be routed with next indexed wavelength in the list. This approach is possible only if wavelength converters are placed at or

13 13 near the mesh since it is not possible to route the light-path into another wavelength without wavelength converters. Otherwise the connection request has to wait indefinitely for the requested wavelength. This overhead will make the average queuing delay still more. Since wavelength converters are found to be expensive, this research work focuses on wavelength assignment without wavelength converters. 1.8 OBJECTIVES OF THIS WORK Fault identification and localization is an important network management function in optical WDM network, which has its impact on survivability of the network. Hence, the main objective of carrying out this research work is to employ fast and effective fault management algorithm, which detect and locate the failures in a network. The second objective is to develop a hybrid adaptive load shifting algorithm for fault recovery with reduced blocking probability and minimum end-to-end delay, thereby increasing the channel utilization and throughput. Therefore, the suggested technique eradicates the problems identified in the conventional technique and achieves a fast and effective fault recovery mechanism in the network. The third objective addresses the flooding issues. With the view of maintaining correct routing tables, the flooding messages have been broadcast to every node in the network. When every node in the network floods the messages, it leads to problems of excessive redundancy of messages, resource contention and signal collision. Since the occurrence of contention is an important and burning issue in optical networks, an efficient flooding technique is required to overcome the flooding issues addressed earlier.

14 14 The fourth objective is to focus on wavelength assignment problem. Here, a Multilevel Feedback Queue Wavelength Assignment (MFWA) algorithm is suggested to achieve balanced utilization of wavelengths which will reduce the connection drop rate for any session request and ultimately route the data successfully to the destination. 1.9 PROBLEM FORMULATION Many researchers have studied the use of dynamic WDM network environment in core optical networks with focus on survivability issues and addressed the problem of efficiently provisioning the light-paths with different protection requirements. In general, it is viewed that the fault detection and localization systems that are practiced at present focus only on the centralized fault management agent in which the agent communicates with all the network nodes through a reliable channel. There is a possibility of overhead for agent and agent failure in centralized management. The static or protection method involves high blocking probability and fast restoration time while the dynamic or restoration method has efficient capacity utilization and slow restoration time. In survivable optical WDM network, after the faults are detected and recovered, the network topology and load changes has to be constantly configured for correct light-path decisions to be made further. The flooding messages are broadcast to all nodes to maintain correct routing tables. When these messages are broadcast to neighbouring nodes of the network, it leads to high protocol overhead and interference with the existing traffic in the networks. Based on survivability requirement of the network, various routing and wavelength assignment schemes face non-uniform distribution of wavelengths and increases the blocking probability of the traffic.

15 15 This work is aimed to employ fast and effective fault management algorithm, which will detect the failures and guarantee proactive fast rerouting. In addition, an efficient flooding algorithm is developed to reduce the flooding overhead, thereby preventing the blocking of forthcoming calls. Non-uniform utilization of wavelengths that leads to blocking probability for any session request will be overcome by developing a novel wavelength assignment algorithm. Performance analysis on the existing techniques is also studied. Evaluation of this work is based on theoretical and performance analysis THESIS ORGANIZATION This thesis deals with the novel fault management technique for network survivability in optical WDM networks to enhance the uniform utilization of resources. This proposes a theoretical and performance analysis based on which results were obtained and simulation has been carried out to ensure the hypothesis. The robustness of this system towards the contention has also been studied and a remarkable improvement is obtained. Chapter 2 presents a detailed literature review in the area of fault management, flooding issues, resource optimization and wavelength assignment for optical network. Chapter 3 describes the distributed fault detection and localization algorithm based on artificial neural network and performance analysis by incorporating the statistical analysis with the existing scheme. Chapter 4 presents the hybrid adaptive load shifting mechanism for fault recovery, where different forms of fault recovery techniques have also been depicted. The simulation results are compared and contrasted in detail.

16 16 Chapter 5 describes the efficient flooding algorithm for improving the network performance to solve flooding issues. The various performance metrics such as flooding messages, network blocking rate, throughput and flooding frequency are considered for this analysis. Chapter 6 presents the multilevel feedback queue wavelength assignment algorithm for achieving balanced utilization of wavelengths. Chapters 7 summarize the contribution of the thesis by comparison and contrast of all the above techniques for optimization of resources through fault management in optical WDM networks. Suggestions for future work are also included.